As a method for producing a perfluoroalkanesulfonic acid represented by the general formula: CnF2n+1SO3H, in which n is an integer of 1 to 3, the method described in U.S. Pat. No. 2,732,398 (Patent Document 1) is known. In short, a perfluoroalkanesulfonic acid is produced by a process by which an alkanesulfonyl halide having 1 to 3 carbons is used as the starting material and is subjected to an electrochemical fluorination reaction in hydrogen fluoride, thereby substituting the hydrogens of the alkyl moiety of the alkanesulfonyl halide with fluorine (electrochemical fluorination) to yield a perfluoroalkanesulfonyl fluoride, and the perfluoroalkanesulfonyl fluoride is then reacted with an alkaline solution to convert the perfluoroalkanesulfonyl fluoride into its alkali metal salt, and further the alkali metal salt is subjected to acid decomposition reaction with sulfuric acid.
In the above-mentioned process, however, it is necessary to collect, at low temperatures, the gas of the perfluoroalkanesulfonyl fluoride generated in the electrochemical fluorination step. Furthermore, it is necessary to react the resulting perfluoroalkanesulfonyl fluoride with the alkali at high temperatures and at high pressures. For these reasons, this process has difficulties, for example, in continuous production and is problematic in industrial implementation.
As an improved production method of the process described above, Japanese Unexamined Patent Application, First Publication No. S64-61452 (Patent Document 2) describes a process by which the production gas resulting from electrochemical fluorination is absorbed while converting into a potassium salt, by enhancing the contact between the production gas and an aqueous solution of potassium hydroxide to react the production gas with the aqueous solution at ordinary pressure. This process is characterized in that a potassium perfluoroalkanesulfonate is crystallized from the gas absorbed solution by concentrating the gas absorbed solution or by adding an alkali to the gas absorbed solution, and then is subjected to filtration and the filtrate is recycled in the gas absorption step.
In this process, however, the crystallization of potassium perfluoroalkanesulfonate is carried out under conditions where potassium hydroxide and potassium fluoride, which is yielded as a byproduct in the reaction, are present in their dissolved state in the gas absorbed solution. As a result, potassium perfluoroalkanesulfonate crystals tend to have the potassium hydroxide and potassium fluoride included therein, which are difficult to remove by washing with water after filtration, since they are incorporated into the inside of the crystals. Therefore, it is not easy to reduce the contents of these impurities to a sufficient degree.
For example, in the case where the potassium perfluoroalkanesulfonate contains potassium fluoride in high amounts, hydrogen fluoride is yielded as a byproduct when a perfluoroalkanesulfonic acid is produced by subjecting the potassium perfluoroalkanesulfonate to an acid decomposition reaction, and results in corrosion of reactor materials, such as glass linings, leading to serious industrial problems.
Also, in the case where the potassium perfluoroalkanesulfonate contains potassium hydroxide in high amounts, water is yielded as a byproduct when a perfluoroalkanesulfonic acid is produced by subjecting the potassium perfluoroalkanesulfonate to an acid decomposition reaction by the addition thereto of concentrated sulfuric acid or the like, and results in the formation of a hydrate or hydrates having a high melting point from the water and the perfluoroalkanesulfonic acid, leading to the problem of clogging pipes during the distillation of the perfluoroalkanesulfonic acid under reduced pressure.
Moreover, process steps become complicated, since the filtrate after the filtration of potassium perfluoroalkanesulfonate in this process has large amounts of potassium perfluoroalkanesulfonate which is the target and unreacted potassium hydroxide remaining therein and is required to be recycled and reused in the gas absorption step.
In addition to the above-described methods, there is an alternative method, Japanese Patent No. 3,294,323 (Patent Document 3) which describes a process by which a methanesulfonyl halide is used as the starting material and is subjected to electrochemical fluorination in anhydrous hydrogen fluoride so as to generate trifluoromethanesulfonyl fluoride, and the trifluoromethanesulfonyl fluoride is then washed with water to remove acidic gases, and is reacted with an aqueous solution or slurry of lithium hydroxide to remove lithium fluoride yielded as a byproduct, thereby to produce lithium trifluoromethanesulfonate. The lithium trifluoromethanesulfonate can be subjected to acid decomposition reaction so as to produce a perfluoroalkanesulfonic acid. In this process, however, an aqueous solution of lithium hydroxide is used as an alkaline absorption solution and lithium fluoride which is sparingly soluble in water is formed as a byproduct, resulting in the problem of depositing within and clogging pipes.
In addition, in any of the methods described above, it has been difficult until now in the electrochemical fluorination step to avoid yielding as byproducts fluoroalkanes and sulfonyl difluoride due to decomposition in the electrochemical fluorination reaction. Global warming recently poses problems, and fluoroalkanes yielded by a decomposition reaction, which is a side reaction in the electrochemical fluorination, are a class of greenhouse gases which are key components of global warming and a group of compounds which have highest coefficients among the presently known greenhouse gases, with their warming coefficients being several thousand times that of carbon dioxide. These fluoroalkanes are not absorbed in acidic or alkaline aqueous solutions, making their posttreatments difficult, and therefore reduction of their incidence is required. The previous production technologies described above do not take into account the suppression of the generation of fluoroalkanes as byproducts.
Also, the sulfonyl difluoride yielded as a byproduct in the decomposition reaction forms potassium sulfate when absorbed in an aqueous potassium hydroxide solution, which has a relatively low solubility in water and thus tends to precipitate within the apparatus in the gas absorption step, leading to the problem of clogging pipes. Furthermore, as the decomposition reaction is enhanced, the content of potassium sulfate in the resulting potassium perfluoroalkanesulfonate crystal is increased. In the case where such potassium perfluoroalkanesulfonate is used as the starting material to carry out acid decomposition reaction for synthesizing a perfluoroalkanesulfonic acid, a reaction residue after the distillation of the perfluoroalkanesulfonic acid under reduced pressure solidifies and makes processing difficult.
Patent Document 1 U.S. Pat. No. 2,732,398
Patent Document 2 Japanese Unexamined Patent Application, First Publication No. S64-61452
Patent Document 3 Japanese Patent No. 3,294,323